1.5.5 ECG lead Systems
The process of recording an ECG is discussed in detail in Chapter 4. Over the years a number of lead systems have been developed to record the cardiac electrical field. The aim of these systems is to record each of the waveforms and complexes clearly so that the conduction process can be evaluated. They may also provide information about the direction and magnitude of the cardiac vecEinthoven's triangle is a system of limb leads which records electrical activity which reaches the body surface in the horizontal (frontal) plane. The heart is assumed to be located approximately in the centre of an equilateral triangle formed by the two forelimbs and the left hind limb. This is the system which is commonly used in small animals and humans. In humans, additional chest leads are useful for detecting areas of abnormal conduction caused by infarcts, a common problem in this species. Chest leads are also useful in some situations in small animals, particularly in animals with congenital heart disease resulting in marked ventricular hypertrophy.
The Einthoven limb lead system can be used in horses and provides useful information about cardiac rhythm and conduction. However, other systems have also been designed to accommodate the fact that in horses the heart is not a point source situated at the centre of a homogeneously conducting triangle formed by the limbs. These systems try to assess the cardiac vector in three dimensions by measuring the electrical field in three orthogonal planes at similar distances from the heart.